Suspension of solid impurities in wet process phosphoric acid



United States Patent 3,408,162 SUSPENSION 0F SOLID IMPURITIES IN WET PROCESS PHOSPHORIC ACID William A. Satterwhite and Robert C. Mazurek, Lakeland Fla., assignors, by mesne assignments, to Armour Agricultural Chemical Company, a corporation of Delaware No Drawing. Filed Feb. 14, 1966, Ser. No. 527,043

6 Claims. (Cl. 23-165) ABSTRACT OF THE DISCLOSURE Wet process orthophosphoric acid contains metal impurities which tend to settle out on standing and also when the acid is concentrated to polyphosphoric acid metaphosphate solids are formed. To prevent such precipitation and build-up of solids, lignosulfonate is mixed with the wet process acid at a temperature of about 80150 C.

This invention relates to the suspension of solid impurities in wet process phosphoric acid.

Undissolved solid impurities in wet process phosphoric acid consist primarily of gypsum, iron and aluminum phosphates and alkali fluosilicates. These impurities settle out in processing, storage, and shipment, and form a hard voluminous mass which is difiicult and costly to remove. Further, such solids are commonly washed out of the containers and discarded, thus representing a substantial loss of P 0 values. The problem becomes more acute when the feed acids are concentrated by evaporation of water to concentrated acids such as, for example, 54% P 0 wet process phosphoric acid. The solid impurities are increased during the concentration operation and precipitate during such processing and during subsequent storage, etc., the impurities forming minute crystal nuclei which grow and agglomerate and deposit in the container, etc. 7

We have discovered that by adding a relatively small amount of alkaline salt of lignosulfonic acid to the Wet process phosphoric acids, solids which normally precipitate with time are retained in suspension, thus effecting considerable savings during handling and in preventing loss of P 0 values.

A primary object, therefore, of the invention is to provide a process for maintaining solid impurities in suspension and inhibiting subsequent growth of such impurities as crystals from supersaturated acid solutions. Another object is to provide a method for dispersing and holding in suspension undissolved solid impurities such as gypsum, iron and aluminum phosphates and alkali fiuosilicates, etc. A further object is to provide a process for treating concentrated acid at an elevated temperature with an alkaline salt of lignosulfonic acid for inhibiting precipitation of solid impurities containing P 0 values, Other specific objects and advantages will appear as the specification proceeds.

In one embodiment of the invention, we add a small amount of lignosulfonate in solid or solution form to west process acid containing 2956% P 0 or higher at a temperature in the range of 80-150" C. or higher. The lignosulfonate is mixed with the solution in such a manner as to coat the solids and thus inhibit crystal growth and agglomeration.

The temperature, time, and amount of lignosulfonate mixed with the phosphoric acid depend upon the amount of solids presentand the extent of size and agglomera tion of the solids. For wet process phosphoric acid containing not more than 5% solids, we find that about 0.1-l.0% lignosulfonate is sufficient for inhibiting crystal growth and agglomeration. For wet process phosphoric acid containing more than 5% solids, a larger amount of Patented Oct. 29, 1968 lignosulfonate up to about 2% or more may be required.

For optimum results, we prefer to add the sulfonate over a period of from 5-15 minutes to the acid at a temperature of about 120150 C. This is suflicient time to assure complete and rapid dissolution of the lignosulfonate. At this temperature, the acid has dissolved a maximum quantity of impurities and agglomerates are minimized. As the acid cools, the impurities crystallize and we find that the minute crystal nuclei are coated by the negatively-charged lignosulfonate additive. The imparted charge produces a repulsion between one particle and another and both agglomeration and crystal growth are inhibited.

As a specific example, wet process phosphoric acid containing about 54% P 0 and treated with lignosulfonate as described above was centrifuged and the solids examined. Photomicrographs of the solids showed the lignosulfonate coating and the inhibition of agglomerates and crystal growth as compared with solids separated from untreated wet process phosphoric acid.

While we prefer the optimum temperatures of 120- 150 C., the use of such temperatures is not practicable in plants using vacuum evaporation equipment wherein a temperature of -100 C. may be a maximum, and longer heating periods may be required for efficiently incorporating the lignosulfonate. For example, at about 88 C., from two to four hours may be required to achieve effective suspension of the solids.

As stated above, the amount of the lignosulfonate required is dependent upon the quantity of solids occurring in the acid, and since the range of solids contained in wet porcess phosphoric acid varies widely, it may be necessary to employ lignosulfonate in a quantity of from 0.5% to as high as 2% by weight. For wet process phosphoric acid having a solids content of about 2.5 to 5.0%, it is found that from 0.5-1.0% by weight of the lignosulfonate is sufficient, while acids containing solids below about 2.5% may require an estimated 0.10.5% of the lignosulfonate.

Any alkaline salt of lignosulfonic acid may be employed. Examples are the sodium, calcium, ammonium and potassium salts. We prefer to employ the sodium salt in providing the lignosulfonate treating agent.

Specific examples illustrative of our process may be set out as follows:

Example I A wet process phosphoric acid containing 5% solids by weight was divided into a number of samples and each sample treated with sodium lignosulfonate varying from 0 to 1.0% and heated to 140 C. Samples of each mixture, along with an untreated control sample, were placed in cm. settling columns. After nine days standing, the control column contained 25 cm. sedimentation. Samples with less than 0.4% lignosulfonate showed sedimentation had occurred but less than in the control. With as low as 0.05% dispersant and with 16 cm. sedimentation, the mass was loose and flowed out of the column when the column was inverted, whereas with the untreated control the sedimentation was very hard and would not flow. At about 0.4%, about 5 cm. sedimentation was observed. At 1.0%, no sedimentation was present.

Example II Tests were carried out as described in Example I except that lower temperatures in the range of 8090 C. were employed. The effects of temperature on the degree of dispersion achieved in very significant. Settling tests were carried out in 38 cm./h. x 5 cm./d. columns. After five days standing, each column was sampled. Nearly equal increments were decanted from the top to the bot- 3 Each i c emen a a a yze for. soli s cont t n s9 ds. Pr ce equ pm ptand. in h ssing. results compared to those with the untreated control. The and avoids the losses arising out of their r effect of time heating at 88"v C., followed by five days equipment cleaning. Since .Lthe solids contain 2045% settling, is shown in the following table:

TABLE I I [Solids suspension in wet process phosphoric acid 1% lignosulfonate additions] r: 10 Minutes Heating 240 Minutes Heating U ntreated Control n Column Increment i v Increment, Solids, Increment, Solids, Increment, Solids,

Wt. Gms. Wt. Gms. Wt. Gms. Wt. Gms. 5 Wt. Gms. Wt.- Grns."

150 00 2. 45 109 00 4.51 161.30 2.58 177 00 3.95 169 00 4.71 171.40 3.00 p 100 50 4.45 155 00 5.54 169.80 3.23 158 50 5.00 170 50 4. 93 152,70 5.09 174 50 10.05 105 40 5.27 173.90 11138 .1 The dispersant at 10 minutes heating is only slightly P 0 the suspension of such solidsrlepres eiits amajor efiective as compared with the control. Whereas, after saving. 240 minutes heating, the dispersant is very effective. While, in the foregolng specification, wehaye setout The amount of the lignosulfonate required is largely procedure inconsiderable detailfor the purpose of ill usdependent upon the quantity of solids in the acid. trating embodimentsof the inv tion, i ll e U de stood that such details may be varied widely by.tho,s;

Examp 16 HI skilled in the art .withouLdepartingfrom the spirit of Tests were carried on as described in Example II. A our invention, sample wet process phosphoric acid containing 2.4% We claim: solids and 0.5% lignosulfonate was found sufficient to 1, In a process for treating-impurity-containing'wet effect suspension. In this test, both the untreated control process phosphoric acid for maintaining impurities'thereand the treated acid were settled in columns for 5 days. in in suspension, the step of mixi'ng'about (ll-2% by The columns were separated into seven increments from weight of lignosulfonate with said wet process phosphoric the top to the bottom. The results are given in Table II acid. below: 2. The process of claim '1 in which the acid is heated TABLE 11 [Solids suspension in wet-process phosphoric acid 0.5% lignosulfonate addition (Solids Less than 3%) 88 C. for 4 hrs. followed by 5 days settling] Without Dispersant Addn. With Dispersant Additions The results of these tests show effective dispersion of in the range of about 8O-l50 C..-and the lignosulfonate the solids in wet-process phosphoric acid. In this case, as mixed with the acid at said temperature.

well as others, it has been noted that a fraction of the 3. The process of claim 1 in whichthe vWet process solids are sequestered by the dispersant. phosphoric acid is heated-t0 about 120-150 ;C.

The control acid contained 2.8% solids; after treat- .4. The process of claim V1 in whichthe .wet process ment with the dispersant the acid is reduced to 2.0% phosphoric acidand lignosulfonate are heated to a temsolids (avg. for seven increments). 1perature of about.80*l00 C. for-a period in excess of an our.

Example IV 5. In a process for treating wet process phosphoric acid to inhibit crystal growth and agglomeration, the steps Suspension tests were carried on as described in Exof heating wet process phosphoric acidcontaining about ample II in an eifort to suspend solids in acids contain- 2,5 5% olid t a temperature of about 8() 1 5Q. C, d ing between 3% and 5% solids. The results are set out mixing lignosulfonate therewithin the amountofabout in the following Table III: O.51% 'by weight. p t

TABLE III Solids suspension in wet-process phosphoric acid 1% lig'nosulfonate additions, 88 C. for 4 hrs. followed by 5 days settling] Acid with 3.1% Solids Acid with 3.8% Solids Acid with 4.4% Solids Column Increment Withont' With Without With Without With D1spers ant Drspersant Dispersant Dispersant. Dispersant .Dispersant Solids, Solids, Solids, Solids, Solids, Solids, Percent Percent Percent Percent Percent Percent 1.6 2.6 0.5 2.8 0.88 2.12 1. 8 2. 7 2. 7 2.8 1.48 2. 88 1. 9 3. 4 3.1 3. 4 3.98 3.15 3.1 2.0 4.0 3.9 0.12 4.40 7.0 3.5 8.1 5.7 9.68 4.99 t i The use of lignosulfonates as described in the forego- 6. The process of claim i in which thellignosulfonate mg examples represent an lnexpensive, simple and diis sodium lignosulfonate. I l

rect means for handling the solids. The process eliminates the handling problem caused by the settling of these 7 (References on following 5 6 References Cited OTHER REFERENCES UNITED STATES PATENTS Baum et al., Chem. Abstracts, Vol. 57, p. 10076b October 1962. 2,929,777 3/1960 Clevenger 23-165 3 103 003 10 19 3 i et 1 106 111 5 OSCAR R. VERTIZ, Przmary Exammer.

3,118,730 1/1964 Nickerson 23-165 A. GREIF, Ass stant Examiner. 

